This invention relates to a control circuit for a variable reluctance motor. More particularly, it relates to a control circuit which utilizes an EMF induced in the winding of a variable reluctance motor to provide information concerning the position of the rotor to permit ampere-turns to be supplied to the motor winding during intervals of increasing magnetic permeance in its magnetic circuit, thereby, to produce motoring torque and continuous motor rotation.
Variable reluctance motors are well-known in the prior art and various exemplary designs of such motors may be seen in U.S. Pat. Nos. 3,700,943 to Heintz; 3,700,944 to Heintz et al; 3,714,533 to Unnewehr; and 3,401,288 to French. These brushless motors employ an exciting winding and a magentic circuit to produce mechanical torque which is substantially proportional to the square of the winding ampere-turns and to the time rate of change of permeance (reciprocal of reluctance), which is a function of the displacement of the rotor in the motor. Typically, these motors employ a stator containing a motor winding and a rotor containing ferro-magnetic elements spaced from one another. Displacement of the rotor relative to the stator produces a variation in reluctance, and, hence, permeance of the magnetic circuit of the motor winding. Of course, displacement of the rotor relative to the stator also produces a variation in the self-inductance of the motor winding, this self-inductance being directly related to the permeance of the magnetic circuit.
The torque or force produced by a variable reluctance motor is proportional to the product of the square of the winding ampere-turns and the rate of change of permeance as a function of rotor displacement. From the preceding, it is apparent that motor torque or force that is positive with respect to some arbitrary reference can only be developed when winding ampere-turns are sustained during an interval in which the permeance increases with rotor displacement. Conversely, negative motor torque or force is developed when winding ampere-turns are sustained during an interval in which the permeance decreases with rotor displacement. Thus, in order to secure continuous rotation of the variable reluctance motor, it is necessary to apply ampere-turns to the motor winding during intervals of increasing permeance and to decrease or eliminate such ampere-turns during intervals of decreasing permeance.
From the above discussion, it is apparent that the winding of a variable reluctance motor must be excited from a time varying source. Furthermore, the time variations of the source must be synchronized with the mechanical rotation of the machine rotor so that winding current is supplied to the motor during intervals in which the permeance increases with displacement and so that such current is interrupted during the intervals in which the permeance is decreasing with displacement. When a timeinvariant source of electrical energy, such as a direct current source, is used, a controller is required to produce synchronized pulsations of winding ampere-turns.
Control circuits for variable reluctance motors in the past have utilized an external position sensor to determine the onset of each of the intervals of increasing magnetic permeance. In U.S. Pat. No. 3,673,476 to D. R. Hamburg, a signal producing apparatus for use with a three-phase variable reluctance motor is described in detail. French Pat. No. 3,401,288 mentioned above, also discloses a position sensing apparatus for a variable reluctance motor. U.S. Pat. Nos. 3,321,685 to Johannes and 3,466,519 to Platnick also disclose position sensing apparatus for use with motors, although the motors are not of the variable reluctance type.